Method for reducing core losses in silicon steels and the like

ABSTRACT

Method for reducing core losses in silicon steels and other similar materials having antiparallel domain wall structures by applying high frequency (e.g., 400-800 Hz.) oscillatory wave energy to a winding surrounding a transformer or other device using the steel or the like as a core material. Following a power surge experienced by the transformer or other electrical apparatus, the number of domain walls is reduced, increasing core losses. By applying a pulse of high frequency wave energy following the surge and at a flux density preferably in excess of 15,000 gauss, the number of domain walls can again be increased to reduce core losses.

United States Patent 1 1 [111 3,716,763

Home, Jr. et al. v [4 1 Feb. 13, 1973 541 METHOD FOR REDUCING CORE 2,820,632 1/1958 Fowler ..317/157.5 LOSSES IN SILICON STEELS AND THE 3,588,624 6/1971 Marvin ..317/157.5

LIKE 7 Primary Examiner-L. T. Mix

[75] Inventors Gerald L. Houze, Jr., Sarver; Merlin Atwmey vincem Gioia et aL L. Osborn, Saxonburg, both of Pa.

[73] 'Assignee': Allegheny Ludlum Industries, lnc., [57] v ABSTRACT Pittsburgh, Method for reducing core losses in silicon steels and [22] Filed: D 29 1971 other similar materials having antiparallel domain wall I structures by applying high frequency (e.g., 400-800 PP 213,585 Hz.) oscillatory wave energy to a winding surrounding a transformer or other device using the steel or the 52 us. 01 ..317/1s7.s 307/101 like as a we maleriaL Folmwmg a surge 51 1111. c1. 111011 13/00 Pe'iewed by "ansfmmer e'ecmcal [58] Field M Search 7/157 307/101 paratus, the number of domain walls 15 reduced, in-

creasing core losses. By applying a pulse ofhigh frequency wave energy following the surge and at a [56] 7 References Cited flux density preferably in excess of 15,000 gauss, the 1 UNTED STATES PATENTS number of domain walls can again vbe increased to reduce core losses.

2,334,593 ll/l943 Wyckoff 2,355,940 8/1944 Zuschcag .:..::.317/157.5 6Claims,4Drawing Figures if x N v 32 24 4: N 4k OSC/LLATOR AMPLIFIER 400 600 z T 'WQ 3,716,763

DIRECTION 0F I ROLL/N6 AND MAGNET/Z4 r/0/v as (75 3a OSCILLATOR 400-80011:

-, OSCILLATOR AMPLIFIER Hz METHOD FOR REDUCING CORE LOSSES IN SILICON STEELS AND THE LIKE BACKGROUND OF THE INVENTION As is known, in the manufacture of grain-oriented iron-silicon alloys, rolling and heat treating techniques are used to align most of the grains with their cube edge parallel to the direction of rolling. This produces a structure in each grain of aligned domains separated by domain walls (i.e., anti-parallel domain wall structures).

It has been found that when a material of this type is subjected to a cyclic magnetic field, such as that produced by a surrounding coil energized by an alternating current source, the domain walls will move back and forth as the field v is periodically reversed. Core losses in such materials, which are divided into hysteresis losses and eddy current losses, can be equated to the energy required to move the domain walls back and forth as the applied field is reversed. In this respect, the core loss associated with any one domain wall is proportional to the square of the rate of movement [i.e., (velocity)] of that wall as the field is reversed.

' Furthermore, as the number of domain walls increases for a given grain size, the distance through which the domain walls must travel as the field is reversed reduces, as does their velocity. Hence, it is desirable to maximize the number of domain walls for a given grain size to minimize core losses.

It has been found that the domain wall spacing in grain oriented silicon steel varies inversely with frequency, i.e., as the frequency increases so does the number of domain walls. Furthermore, if the steel is demagnetized (by smoothly decreasing the amplitude of the magnetizing force from a value sufficient to induce magnetic saturation to zero) at a high frequency, e.g., 400 Hz., the refined domain spacing is essentially metastable at a lower frequency, e.g., 60 Hz., provided that the peak induction at the lower frequency is kept at or below about 15,000 gauss. Therefore, core losses can be reduced by taking advantage of this domain spacing refinement.

A transformer or similar device with a silicon steel core experiences constant core losses whenever it is connected to the line regardless of load conditions. This means that over the life of a piece of equipment even a few percent reduction of the core losses represent a substantial cost saving.

SUMMARY OF'THE INVENTION The present invention resides in the discovery that core losses can be reduced in the core of a transformer or other electrical device by momentarily exciting the transformer core with high frequency alternating current. The invention finds particular utility with electrical devices operated at about 60 Hz. By applying a high frequency pulse of about 400-800 Hz. at a flux density in excess of 15,000 gauss to a transformer, the number of domain walls for a given grain size actually increases. Furthermore, it has been found that the domain spacing refinement associated with high frequency excitation of this type is metastable and will be retained at lower operating frequencies once established, thereby decreasing core losses. Pulsed excitation below 400 Hz.

and above about 1,000 Hz. does not result in the desirable increase in domain walls, nor does a reduction in core loss occur. Likewise, the desirable results of the invention are not achieved unless the core material is pulsed at the high frequency at a flux density of 15,000 gauss or above, preferably at least 17,000 gauss.

In principle, a transformer core could receive a high L frequency demagnetizing treatment at the time of its manufacture to establish a fine domain structure. However, once such a transformer core were placed in service its domain structure could be coarsened by the high peak inductions associated with momentary overloads, lightning, etc. Therefore, a demagnetizing device should be built into the transformer to restore the low loss domain configuration after such incidents as described above are experienced.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. I is an enlarged view of a typical grain structure of grain-oriented silicon steel showing the formation of domains therein;

FIG. 2 illustrates a single grain of grain-oriented silicon steel or the like showing the manner in which domain boundaries shift back and forth under the influence of an alternating current magnetic field and the generation of eddy currents therein;

FIG. 3 is a schematic circuit diagram showing the demagnetizing apparatus of the invention; and

FIG. 4 is a schematic circuit diagram showing the manner in which the test results hereinafter set forth in this specification were derived.-

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to the drawings, and particularly to FIG. 1, there is shown an enlarged view of the grain structure of grain-oriented silicon steel which includes six grains G through G Only the grain G is shown in its entirety. The direction of rolling a material of this type is indicated by the reference numeral 10. This is also the direction in which the magnetic properties of the material are best (i.e., the direction of easiest mag netization).

In accordance with the well-known characteristics of grain-oriented silicon steel, a plurality of domain regions extend along each grain in the general direction of rolling. For example, in grain 6,, adjacent domain regions are identified as 12 and 14, alternate ones 12 of the domain regions being darker in appearance than the intermediate regions 14. Furthermore, each of the regions 12, for example, is magnetized with a polarity opposite to that of the regions 14; however the fields produced by the respective regions cancel each other so that a strip of silicon steel, for example, appears unmagnetized.

The domain regions 12 and 14 are separated by domain walls 16. When a strip of grain-oriented silicon steel of this typs is subjected to an alternating magnetic field, the effect is to shift the domain walls 16 between domain regions back and forth. This is shown, for example, in FIG. 2 where the arrow 18 shows the back and forth motion of the domain wall 16 and the dotted lines 16 show the extent of movement of the domain wall. When the domain wall 16 thus moves back and forth under the influence of an alternating magnetic field, eddy currents 20 are generated around the domain wall 16. In this respect, the core loss of a metal having an antiparallel domain wall structure such as that shown in FIGS. land 2 can be defined as the energy required to move the domain walls back and forth -under the influence of the alternating magnetic field.

Furthermore, the magnitude of the core loss is dependent upon the square of the velocity of the domain wall. Hence, at a given frequency the further the domain wall must move, the faster it must move, and the greater the core loss. As the number of domain walls decreases, the distance through which a domain wall must move during each reversal of flux increases. Hence, as the number of domain walls decreases, the core losses increase. Similarly, as the number of domain walls increases, the core losses decrease.

As was explained above, the domain spacing in oriented silicon steel is inversely related to excitation frequency, but that fine domain spacings established at high frequencies are metastable at a lower frequency. This effect can be utilized to obtain lower losses in a transformer core. However, if thetransformer experienced a current surge at, e.g., 60 Hz., the fine high frequency domain spacing would be destroyed and a coarser spacing characteristic of 60 Hz. excitation established. Therefore, in order to obtain the maximum benefit from domain spacing refinement, a high frequency demagnetizing device should be built into the transformer.

The demagnetizing device must be capable of producing the'high frequency for a short period of time and the magnetizing force be reduced smoothly to zero. This device might be a power source and control connected to the transformer winding at theproper time or a resonance circuit consisting of the transformer winding, core and capacitance of such value as to resonate at the desiredfrequency. An oscillatory discharge from this circuit would perform the demagnetization function. I

Apparatus for applying high frequency current to a transformer is shown schematically in FIG. 3. The transformer core is indicated by the referencenumeral 22 and is provided with a primary winding 24 and a secondary winding 26 connected through switch 28 to a load, R The primary winding 24 is connected through a circuit breaker 30 to a source of alternating current voltage '32 typically having a frequency of about 60 Hz. Should the transformer core 22 experience a current surge, causing a flux density in excess of about to 17,000 gauss, a reduction in the number of domain walls occurs as explained above. In order to again increase the number of domain walls, the circuit breaker 30 is opened and the primary winding 24 is connected through switch 34 and amplifier 36 to an oscillator 38 having a frequency in the range of about 400-800 Hz. for the case where 60-cycle power is normally applied to the primary winding.

' 1n the practice of the invention, the switch 34, with circuit breaker 30 open, is momentarily closed to connect the amplified high frequency wave energy across the primary winding 24. The resulting high frequency pulse or burst is of sufficient amplitude to saturate the antiparallel domain movement in the core, about 10 oersteds. The amplitude of the high frequency pulse is then quickly and smoothly reduced to zero by reducing the gain of amplifier 36. After opening the switch 34, the circuit breaker 30 can be again closed, connecting the power source 32 to the primary winding 24. The refined domain spacing which reduces core losses established by the demagnetization treatment will persist as long as the operation induction of the core is kept at or below about 15,000 gauss; however should the induction increase above 15,000 gauss, as is experienced under a current surge, the demagnetizing treatment must again be repeated.

The desirable results of the invention were tested with the test apparatus shown in FIG. 4 of the drawings.

A test sample 40, typically having a weight of about 10 grams, a density of about 7.69 grams per cubic centimeter and a total length of about 15 centimeters, is disposed within primary and secondary windings 42 and 44, the magnetic circuit being completed by a yoke system 60. Actually, these windings 42 and 44 are normally wound about each other; however they are shown spaced along the sample 40 in FIG. 4 for purposes of explanation. Adapted to be connected to the primary winding 42 through switch 46 is a source of 60 Hz. alternating current power 48. Alternatively, an oscillator 50 can be connected to the primary winding 42 by closing switch 52. The secondary winding 44 is connected across a voltmeter 54 and is also applied to a wattmeter 56 through a range resistor, the other input to the wattmeter being derived by coils in series with the primary winding.

Samples, such as sample 40, were tested by initially energizing coil 42 byclosing switch 46, the voltage of the source 48 being regulated to produce induction levels of 10,000, 15,000 and 17,000 gauss, respectively. Following magnetization at any induction level, the sample 40 was demagnetized at 60, and 400 cycles, respectively, the core losses after each demagnetization step being observed by noting the millimeter displacement of the wattmeter 56. From a consideration of the watts per millimeter deflection of the wattmeter and the instrument loss, the core loss in watts per pound can be obtained by first multiplying the millimeter deflection times watts per millimeter, subtracting the instrument loss, and thereafter dividing by the active weight of the sample.

The results of such tests are shown in Table 1:

TABLE 1 Core loss (watts/lb.) at a demagnetization frequency Iuduet. of

1evel, Sample gauss 60 c.p.s. 60 120 60 120 400 Number Wt.-lb. X10 voltage Hz. Hz. Hz. Hz. Hz.

6231 10. 112 10 2. 299 309 298 311 298 265 15 3. 449 661 648 661 650 630 17 3. 908 962 958 962 954 958 6245 9. 839 10 2. 237 272 259 263 259 239 15 3. 356 606 593 598 595 589 17 3. 803 878 880 882 882 882 6255 9. 429 10 2. 144 214 217 214 217 207 15 3. 216 482 486 484 484 482 17 3. 645 757 757 762 757 753 6268 9. 627 10 2. 189 337 332 330 328 293 15 3. 284 713 709 716 722 696 17 3. 721 973 973 .981 .981 977 6269 9. 703 10 2. 206 25B 258 258 258 236 15 3. 309 593 591 593 597 593 17 3. 750 884 884 893 884 890 6270 10. 001 10 2. 277 258 262 258 252 232 15 3. 416 565 572 563 570 533 17 3. 871 801 801 806 806 784 6461) '1. 416 10 2. 141 276 269 281 281 238 15 3. 212 601 601 509 594 594 17 3. 6 10 838 832 831i 836 832 [i710 9. 312 11) 2.118 284 294 289 271 257 15 3. 177 662 671i 1372 953 653 17 3, 601 J64 9118 .168 .1164 .1114

Table l- Contmued TABLE I Core loss (watts/lb.) at a demagnetization frequency Induct. of-

level,

Sample gauss 60 c.p.s. 60 120 60 120 400 Number Wt.-lb. X voltage Hz. Hz. Hz. Hz. Hz.

Demagnetizatlon pulses applied for about 1 second at flux level of 17,000 gauss.

Each sample was tested at induction levels of 10,000, 15,000 and 17,000 gauss, respectively. It will be noted that as the induction level increases, so also must the voltage output of the source 48, for example. This is because of the relationship:

B= (V)/K AXNXF where:

B flux density, V= voltage,

K a constant,

A cross-sectional area of the sample,

N number of turns on the secondary winding, and

F frequency of the applied voltage.

From the foregoing, it can be seen that flux density can be increased by increasing the voltage or decreasing the frequency. By the same token, for a given fixed value of flux density, the voltage must increase as frequency increases.

With reference specifically to Table I, it will be noted that for Sample 6231, for example, demagnetization at a frequency of 60 Hz. after energization of an induction level of 10,000 gauss produces very little reduction in core loss. However, when the demagnetization frequency is increased to 120 Hz., the core loss drops from 0.309 watts per pound to 0298 watts per pound. At 400 Hz., the core loss drops still further to 0265 watts per pound. Similar results are produced at an initial induction level of 15,000 gauss with the core loss dropping after demagnetization from 0.661 watts per pound at 60 Hz. to 0630 watts per pound at 400 Hz. demagnetizing frequency. When the induction level is increased to 17,000 gauss, very little reduction in core loss is achieved, regardless of the frequency of the demagnetizing wave energy. Hence, in order to achieve the desirable results of the invention, the transformer or other electricaldevice should be operated at an induction I level no greater than 15,000 gauss. Very similar results are obtained with the other samples identifiedin Table I.

With reference to Table II, the necessity for using a demagnetizing frequency in the range of about 400-800 cycles per second is shown.

3 60 4 .505 4 I20 8 5 400 30 .274 5 s00 60 .254 7 1000 75 .276 8 60 4 297 9 so 4 .303 10 4 .297 1 1 400 30 .282 12 400 so .246 1a 400 so 1 .244 14 400 20 I .267 15 so 4 .305 15 400 10 .291 17 400 so .244 18 800 60 .259 19 800 50 .282 20 s00 50 .265 21 800 so .255 22 1000 70 .272

All of the tests shown in Table ll were conducted on a single strip at an operating induction level of 10,000 gauss at 60 Hz. Note that when the demagnetizing frequency is 60 Hz., core losses close to 0.3 watts per pound result as shown, for example, by Test Nos. 1, 3, 8, 9, l0 and 15. The lowest core loss achieved at a demagnetizing frequency of 60 Hz. is that for Test Nos. 8 and 10 where it is 0.297. When the demagnetizing frequency is increased to 120 Hz. per second, the core loss after the demagnetizing treatment at 60 Hz. and 10,000 gauss induction level is still close to 0.3 watts per pound as evidenced, for example, by Test Nos. 2 and 4 where the core losses are 0.291 watts per pound and 0.289 watts per pound, respectively. However, when the frequency of the demagnetizing current is increased to 400 Hz., the core loss decreases substantially as evidenced by Test Nos. 5, l 1, 12, l3, l4 and 17 where the core losses range from about 0.274 watts per pound down to about 0.244 watts per pound. Note that Test No. 16 was run at a demagnetizing voltage of 10 volts, meaning that the induction level during the demagnetizing treatment was not high enough to effect the desirable domain boundary refinement. As was stated above, the induction level of the demagnetizing pulse must be at an induction level of at least 17,000

gauss.

At a demagnetizing frequency of 800 Hz., the desirable results of the invention are again produced as evidenced by Test Nos. 6,18, 19, 20 and 21 where the core losses are as good as or better than those obtained 50 with a demagnetizing frequency of 400 Hz. However,

when the demagnetizing frequency is increased to 1,000 Hz. as evidenced by Test Nos. 7 and 22, the core losses begin to increase. Hence, the frequency of the demagnetizing pulse should be in the range between about 400 and 800 Hz. at a flux or induction level of at least 17,000 gauss.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In the method for reducing core losses in magnetic materials normally subjected to magnetic flux produced by an exciting frequency of about 60 hertz and having antiparallel domain wall structures, the step of applying to a winding surrounding said material an alternating current having a frequency of at least 400 hertz to thereby increase the number of domain walls in said material and reduce its core loss.

2. The method of claim 1 wherein said wave energy has a frequency in the range of about 400 to 800 hertz.

3. The method of claim 1 wherein said material is normally subjected to magnetic flux produced by a 60 hertz alternating current generator connected to a winding surrounding said material, and including the step of disconnecting said generator from said winding before said alternating current having a frequency of at least 400 hertz is applied to a winding surrounding said material,

4. The method of claim 3 including the step of disconnecting said alternating. current having a frequency of at least 400 hertz from said winding and thereafter again connecting said generator to the winding surrounding said material.

5. The method of claim 1 wherein said magnetic material is used in the core of an electrical device operated at a flux level at or below about 15,000 gauss and said alternating current of at least 400 hertz induces in said material a flux density of at least 17,000 gauss.

6. The method of claim 6 wherein said alternating current of at least 400 hertz is applied momentarily and reduced smoothly to zero. 

1. In the method for reducing core losses in magnetic materials normally subjected to magnetic flux produced by an exciting frequency of about 60 hertz and having antiparallel domain wall structures, the step of applying to a winding surrounding said material an alternating current having a frequency of at least 400 hertz to thereby increase the number of domain walls in said material and reduce its core loss.
 1. In the method for reducing core losses in magnetic materials normally subjected to magnetic flux produced by an exciting frequency of about 60 hertz and having antiparallel domain wall structures, the step of applying to a winding surrounding said material an alternating current having a frequency of at least 400 hertz to thereby increase the number of domain walls in said material and reduce its core loss.
 2. The method of claim 1 wherein said wave energy has a frequency in the range of about 400 to 800 hertz.
 3. The method of claim 1 wherein said material is normally subjected to magnetic flux produced by a 60 hertz alternating Current generator connected to a winding surrounding said material, and including the step of disconnecting said generator from said winding before said alternating current having a frequency of at least 400 hertz is applied to a winding surrounding said material.
 4. The method of claim 3 including the step of disconnecting said alternating current having a frequency of at least 400 hertz from said winding and thereafter again connecting said generator to the winding surrounding said material.
 5. The method of claim 1 wherein said magnetic material is used in the core of an electrical device operated at a flux level at or below about 15,000 gauss and said alternating current of at least 400 hertz induces in said material a flux density of at least 17,000 gauss. 